1. Field
The present disclosure relates generally to inert gas welding. More particularly, the invention is directed to purge dams for retaining purge gas around a weld zone.
2. Description of the Prior Art
By way of background, inert gas welding is a species of arc welding in which the molten weld pool is shielded from atmospheric contamination and oxidation by bathing it with an inert gas, such as Argon, or a mixture of Helium and Argon. Popular examples of inert gas welding include TIG (Tungsten Inert Gas) welding and MIG (Metal Inert Gas) welding.
When welding together pipes and other enclosed structures using inert gas welding, it is important to purge the interior of the pipe or structure in the vicinity of the weld zone to prevent corrosion and the formation of oxides on the interior side of the weld pool. Purge dams are conventionally used for this purpose. For example, when butt-welding the ends of two pipe sections to form a consolidated pipe run, two purge dam structures are placed in the pipes, one in each pipe on either side of the weld zone. A purge gas can then be introduced into the area between the dams.
Water degradable purge dams have been proposed that are made from water degradable paper. The advantage of water degradable paper purge dams is that they can be placed in close proximity to a weld zone, and then removed following welding by dissolving or otherwise degrading them with water introduced through the pipe. Insofar as pipe systems typically undergo hydrostatic pressure testing or flushing with water prior to use, water soluble purge paper can be used in many cases without any additional processing steps following welding. Such purge dams are typically formed from a sheet of water degradable paper that is formed into a concave shape in which a central portion of the purge dam spans across the diameter to the pipe to be welded and a peripheral edge portion of the purge dam engages the pipe wall so that it can be secured thereto, such as by using water soluble tape.
Conventional water degradable purge dam devices as described above are designed for use with pipe installations with little or no air flow within the pipes being welded. However, there are high air flow environments in which conventional water degradable purge dams are not practical because the purge dams cannot withstand the air pressure fluctuations within the pipes. For example, underwater pipelines that are miles in length can develop large bidirectional air pressure surges. Undersea pipeline installation contractors sometimes refer to this phenomenon as “suck and blow.” When fabricating such pipelines, the end of the existing portion of the pipeline is located and pulled up to the surface by a pipe-lay vessel that carries the next run of pipe, which itself may be several miles in length, on a spool. Inert gas welding to connect the ends of the two pipe sections is performed onboard the ship using a welding jig that aligns the pipe sections to form a welding root gap. However, it is first necessary to ensure that the bidirectional air flow within the pipe sections cannot enter the weld zone, where it would quickly displace the purge gas. This is typically done by inserting blocking members, made from a suitable foam material, into the ends of the pipe sections. Unfortunately, this practice has met with mixed success because the blocking member can dislodge if the air pressure fluctuations are large enough, and may be difficult to remove following welding.
Applicants submit that there is presently a need for improvement in the construction of water-degradable purge dams for high air flow environments. What is required in particular is a purge dam apparatus that can be used in long pipelines that develop significant bidirectional air pressure fluctuations therein.